X-Inactivation by Chromosomal Pairing Events

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PERSPECTIVE

X-inactivation by chromosomal pairing events

York Marahrens1,2

1Department of Human Genetics, University of California at Los Angeles (UCLA), Los Angeles, California 90095 USA

X-inactivation is the coordinated silencing of nearly all Dosage compensation is the coordinate regulation of genes on one of the two X chromosomes in female mam- X-linked genes by chromatin remodeling to provide the mals. X-inactivation requires the cis-acting Xist gene. two sexes, which have a twofold difference in X chromo- The highly unusual properties of Xist and the extremely some number, with equal levels of X chromosomal gene long distances over which Xist acts have made it difficult expression. In Drosophila, this is accomplished by large to reconcile X-inactivation with other examples of gene numbers of cis-acting elements, each of which controls a regulation. This paper presents new findings that suggest single gene or a small group of adjacent genes (Baker et that X-inactivation involves transvection and harnesses al. 1994). These elements up-regulate gene expression heterochromatin association. twofold on the single male X chromosome by ‘loosening’ the chromatin of each gene to make it more euchromatic. In Caenorhabditis elegans, proteins associate Role of chromatin in dosage compensation with numerous sites along both of the X chromosomes of Chromatin is the complex of DNA, histones, and other the XX individuals, down-regulating gene expression factors that compose chromosomes. Originally, eukary- twofold by making each X chromosome slightly more otic chromosomes were believed to consist of euchroma- heterochromatic (Nicoll et al. 1997; Dawes et al. 1999). tin, regions that permit gene expression, and heterochro- The chromatin adjustments that produce twofold matin, chromosomal regions that are condensed and re- changes in gene expression in Drosophila and C. elegans pressive to gene function (Heitz 1928; Karpen 1994). In underscore that a spectrum of chromatin structures are general, the DNA of heterochromatin is more heavily possible, spanning from very heterochromatic to very eu- methylated (in organisms that have DNA methylation) chromatic. (Selker 1990; Klein and Costa 1997; Garrick et al. 1998), Dosage compensation in mammals (X-inactivation) is and replicates later in S phase (Taylor 1960) than the fundamentally different from dosage compensation in DNA of euchromatin. The histones of heterochromatin Drosophila and C. elegans. The mammalian cell is are also less acetylated (Turner 1998). Several lines of thought to first ‘count’ the X chromosomes and, if two X evidence indicate that the terms euchromatin and het- chromosomes are present, activate the X-inactivation erochromatin should both be considered umbrella terms pathway. Rather than regulating all of the X-linked genes for multiple chromatin structures. in a cell equally, the cell ‘chooses’ one X chromosome to There are two types of heterochromatin, constitutive be inactivated through the spread of heterochromatin ∼ heterochromatin and facultative heterochromatin. Con- across 160 Mb. The Xist locus is involved in all three stitutive heterochromatin forms over repetitive se- processes, namely counting, choosing, and long distance quence and serves to maintain genome stability. Consti- heterochromatin formation. Xist, therefore, is required tutive heterochromatin suppresses recombination be- for chromatin remodeling over a far greater distance than tween repetitive sequences and subdues the mutagenic any other known cis-acting locus. potential of transposons by keeping them constitutively The Xist gene was first identified when a large (15 kb) inactive (Yoder et al. 1997; Jensen et al. 1999). This is untranslated RNA was cloned that is expressed exclu- important, as ∼35% of the human genome consists of sively from the inactive X chromosome (Borsani et al. repetitive transposon DNA sequence (Yoder et al. 1997). 1991; Brown et al. 1991). Interestingly, the Xist RNA In contrast, facultative heterochromatin is formed and associates exclusively with the inactive X chromosome dismantled in a controlled manner by specific chromatin (Brown et al. 1992; Clemson et al. 1996; Panning and control sequences to regulate gene expression. The abil- Jaenisch 1996). Targeted mutagenesis revealed that Xist- ity of specific chromatin structures to spread to nearby deficient X chromosomes are incapable of being inacti- genes allows gene expression to be regulated. vated in cultured cells (Penny et al. 1996) or during em- bryogenesis (Marahrens et al. 1997). A surprising discovery was the finding that a 450-kb transgene, that 2Corresponding author. includes the Xist locus, could inactivate an autosome E-MAIL [email protected]; FAX (310) 794-5446. (Lee et al. 1996; Lee and Jaenisch 1997). The sequence

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X-inactivation by chromosomal pairing required for Xist transgene-mediated inactivation was random (Penny et al. 1996) and imprinted (Marahrens et subsequently narrowed down to a 35 kb region that con- al. 1997) X-inactivation, demonstrating that the two tains the Xist transcribed region and 9 kb of upstream types of X-inactivation are mechanistically related. sequence (Herzing et al. 1997). Three different parameters indicate that, for both X- The highly distinctive properties of Xist and the ex- inactivation and autosomal imprinting, the allelic re- tremely long distance effects of the X-inactivation pro- gions in question differ in their chromatin structures. cess have made it difficult to reconcile X-inactivation First, extensive differences in methylation have been re- with other examples of gene regulation by chromatin. ported between the active and inactive X chromosomes However, it is highly unlikely that a new and fundamen- (Mohandas et al. 1981; Migeon 1990) and between the tally different biological process has evolved to take care two copies of autosomal imprinted regions (Bartolomei of the X chromosome gene dosage problem in mammals. and Tilghman 1997). Demethylation results in loss of The available evidence overwhelmingly indicates that allelic differences in gene expression for both imprinted basic biological processes are conserved among diverse genes (Li et al. 1993) and for the X chromosome (Beard et organisms. al. 1995; Panning and Jaenisch 1996). Second, the his- In this paper I consider the possibility that the pro- tones of heterochromatin are less acetylated than in eu- cesses responsible for X-inactivation are not unique to chromatin (Grunstein 1997; Turner 1998). The inactive X-inactivation. I first highlight significant functional X chromosome is less acetylated than the active X chro- similarities between genomic imprinting and X-inacti- mosome (Jeppesen and Turner 1993). The treatment of vation which culminate in recent findings by Rolf Ohls- cells undergoing X-inactivation with trichostatin, a his- son and colleagues. In considering these similarities, I tone deacetylase inhibitor presented normal inactivation propose that X-chromosome counting, choosing, and the (O’Neill et al. 1999). Trichostatin treatment of mouse initiation of heterochromatin formation are the genetic conceptuses attempting to establish the monoallelic ex- consequence of physical pairing interactions between pression pattern of the imprinted H19 gene similarly dis- the homologous X chromosomes (transvection). I next rupts the silencing of one allele of the H19 gene (Svens- discuss recent evidence presented by Lyon (1998) that son et al. 1998). Finally, heterochromatin is later repli- indicates the spread of heterochromatin over long dis- cating than euchromatin and chromatin has been shown tances is brought about by interactions involving repeti- to be an important determinant of replication origin tim- tive elements distributed throughout the X chromo- ing (Fangman and Brewer 1992). Both imprinted genes some. In light of recent findings by Gartler and col- (Kitsberg et al. 1993) and X-chromosomal genes in fe- leagues (1999), Kominami and colleagues, and Henikoff males (Ohno and Hauschka 1960; Taylor 1960) are early and coworkers, I suggest that heterochromatin associa- replicating on one homologous chromosome and late tion is harnessed to control gene expression levels by replicating on the other. heterochromatin spreading. Finally, I show how this new For some examples of autosomal imprinting and X- view of X-inactivation can be adapted in a straightfor- inactivation, chromatin differences between the ho- ward manner to explain reports of skewed X-inactiva- mologous chromosomes span large regions. Many, but tion. The mechanisms discussed are useful in under- not all imprinted genes are grouped into megabase-sized standing a rapidly increasing number of characterized clusters. Some mutations within these gene clusters ex- genetic diseases. hibit long distance effects on gene expression. One of the best studied imprinted clusters is on human chromosome 15 and is implicated in both Prader-Willi syndrome (PWS) and Angelman syndrome (AS) (Mutirangura et al. Similarities between autosomal imprinting 1993; Driscoll 1994; DeLorey et al. 1998). Some PWS and and X-inactivation AS patients carry mutations in this imprinted gene clus- Important clues explaining how X-inactivation might be ter that influence imprinted gene expression and/or initiated can be obtained from studies of autosomal im- DNA methylation patterns across hundreds of kilobases printing. Imprinted gene expression is determined by the and, apparently, even across megabases (Glenn et al. sex of the parent from which an allele is inherited. Im- 1993; Sutcliffe et al. 1994). Therefore, the imprinted au- printed genes are either exclusively silent on the mater- tosomal regions share with the X chromosome, a capac- nal chromosome or are exclusively silent on the paternal ity to influence gene expression over very long distances. chromosome (for review, see Bartolomei and Tilghman Both X-inactivation and at least one example of im- 1997). Like X-linked genes, imprinted genes are, there- printed gene silencing are regulated by control loci that fore, silent on one of two homologous chromosomes and resemble each other. The autosomal H19 gene (Barto- expressed from the other. lomei et al. 1991; Rachmilewitz et al. 1992; Zhang and A direct relationship between autosomal imprinting Tycko 1992) resembles Xist in several ways (Pfeifer and and X-inactivation was established when it was demon- Tilghman 1994). The phenotypes of Xist and H19 knock- strated that X-inactivation is imprinted in the extraem- out mice strongly indicate that the exclusive roles of bryonic tissue of rodents (Takagi and Sasaki 1975) and in both genes are to silence other genes on the same chro- all tissues of marsupials (Cooper 1971; Sharman 1971). In mosome at one allele (Leighton et al. 1995; Marahrens et these tissues, only the X chromosome from the father is al. 1997). Both Xist and H19 are genes with small introns inactivated. Xist has been shown to be required for both that express untranslated RNAs from the allele that does

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Marahrens the silencing, whereas their promoters on the other al- an imprint on the maternal but not the paternal X chro- lele are repressed and methylated (Ferguson-Smith et al. mosome. However, in the extraembryonic tissue of hu- 1991; Bartolomei et al. 1993; Brandeis et al. 1993; Norris man androgenetic (both sets of chromosomes inherited et al. 1994; Beard et al. 1995). The alleles of H19 and Xist from the father) conceptuses, Ohlsson and coworkers which do not silence other genes are transcriptionally showed recently that both H19 and Igf-2 are randomly silent yet reside in early replicating chromatin. The ex- inactivated (Ohlsson et al. 1999). Allele-specific in situ pressed allele which silences other genes resides in late hybridization was used to show that, in the absence of replicating chromatin (Bartolomei and Tilghman 1997). the distinguishing imprints, the expression of both genes Gene targeting revealed that the H19 transcript is not became variegated, suggesting random inactivation fol- required for the silencing function of H19 (Ripoche et al. lowed by clonal inheritance of the gene expression pat- 1997; Jones et al. 1998). H19, therefore, may not really be terns. Therefore, the maternal, but not the paternal chro- a gene in this respect but rather a control locus for other mosome contains the imprint in human extraembryonic genes that elicits a transcript. Gene targeting has also tissue. This implies that a random inactivation mecha- revealed that Xist functions as a cis-acting control locus. nism (as with Xist in somatic tissue) causes one allele of The similarity of Xist and and H19 suggests a common H19 and Igf-2 to be silenced and that this mechanism is mechanism. However, unlike H19, Xist silences genes rendered nonrandom by imprinting (as with Xist in over extremely long distances and the RNA ‘coats’ the mouse extraembryonic tissues). inactive X chromosome, suggesting it may play a func- If imprinted genes are subject to a random silencing tional role. Therefore, the similarity between the two mechanism that is skewed by a superimposed imprint, genes may be the way they establish differences in the then it is reasonable to expect that autosomal genes, chromatin structures on their own homologous alleles. which are subject to the random inactivation mechanism but lack an imprint, also exist. This was shown to indeed be the case when it was demonstrated that the Random vs. imprinted inactivation: mouse autosomal interleukin-2 (IL-2) gene is randomly A shared mechanism? inactivated (Hollander et al. 1998). The inactive allele of Two lines of evidence suggest that imprinted and ran- IL-2 is late replicating while the expressed allele is early dom choosing of an X chromosome for inactivation pro- replicating, suggesting that chromatin similarly plays a ceeds by the same mechanism. First, X-inactivation can role in IL-2 silencing. The random formation of late rep- be made randomly in tissues where it is normally im- licating chromatin at one allele, therefore, is not unique printed. In conceptuses where both X chromosomes are to X-inactivation even under normal circumstances. inherited from the mother, both X chromosomes are able This finding raises the very real possibility that many to resist inactivation in the imprinted extraembryonic other genes are also randomly inactivated but that this tissue where the paternal X chromosome is normally has gone unnoticed. The striking difference between IL-2 inactivated (Rastan et al. 1980; Endo and Takagi 1981; inactivation and X-chromosome inactivation is, of Shao and Takagi 1990; Tada and Takagi 1992). This im- course, the distance the inactivation spreads; the random printed resistance, however, is incomplete because a inactivation on the autosome appears to be confined to subset of the cells undergo random inactivation. Further- only the one gene (Hollander et al. 1998). more, inheritance of two paternal X chromosomes also results in random X-inactivation (Endo et al. 1982). This suggests that monoallelic Xist transcription is caused by A transvection hypothesis for X-chromosome a random inactivation mechanism which, in extraem- counting, choosing, and the initiation bryonic tissue, is rendered nonrandom by a discriminat- of heterochromatin formation ing imprint. The partial resistance, in parthenogenetic embryos, of both maternal X chromosomes to X-inacti- These results indicate that autosomal gene silencing and vation indicates that the imprint that skews X-inactiva- X-inactivation are related, and that imprinted and ran- tion is on the maternal X chromosome. dom inactivation share a common mechanism. How can The second line of evidence that random and im- one allele be randomly silenced while the corresponding printed choosing are related was provided recently by region on the homologous chromosome remains ex- analysis of imprinted autosomal genes. Imprints that repressed? A possible explanation is the discovery that two strict H19 expression to the maternal chromosome and clusters of imprinted genes physically pair with their Igf-2 expression to the paternal chromosome gene appear homologous counterparts during late S-phase (LaSalle to be present on both maternal and paternal autosomes and Lalande 1996). In contrast, none of several nonim- because both H19 alleles are silent and both Igf-2 alleles printed control regions tested showed homologous pair- are expressed in mouse androgenetic (two paternal sets ing (LaSalle and Lalande 1996). A large deletion in one of chromosomes) embryos (Sasaki et al. 1995) while both copy of the imprinted region disrupted this homologous H19 alleles are expressed and both Igf-2 alleles are silent association (LaSalle and Lalande 1996). These findings in mouse (Walsh et al. 1994) and human (Mutter et al. raise the possibility that homologous association is re- 1993) parthenogenetic (two maternal sets of chromo- quired for the monoallelic expression of imprinted genes somes) embryos. This is in contrast to X-inactivation in in the region. Changes in gene expression, caused by mouse extraembryonic tissue where there is evidence for physical interactions between homologous chromo-

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X-inactivation by chromosomal pairing somes, are known to occur in nonmamalian organisms and are referred to as transvection. Transvection was first discovered more than forty years ago in Drosophila, when a mutant phenotype for the Ultrabithorax (Ubx) gene was observed in flies het- erozygous for a structural rearrangement that relocated Ubx, but not in flies lacking the rearrangement, or in flies homozygous for the rearrangement (Lewis 1954). It had previously been shown that homologous chromosomes physically pair during part of the mitotic cell cycle (Metz 1916) and the mutant phenotype was attrib- uted to the rearrangement disrupting the pairing of the two alleles of Ubx (Lewis 1954). Several additional examples of transvection were described subsequently in Drosophila (Henikoff 1997) and related phenomena have been observed in plants (Matzke and Matzke 1995b; Meyer and Saedler 1996). The observation that imprinted gene clusters in mammals associate homologously, raised questions whether this pairing results in transvection, as it does in Dro- Figure 1. Proposed role of transvection in X inactivation. In sophila. Transvection was recently revealed at one of females, two homologous X chromosomes (blue lines) homolo- these regions using gene targeting (Duvillie et al. 1998). gously pair at certain regions (jagged borders). This pairing may When the paternal allele of the imprinted Ins-2 gene was require stretches of heterochromatin (not shown). The homolo- disrupted in the gene body with a neo expression cas- gous association juxtaposes the two Xist alleles causing one Xist sette, the Ins-2 promoter on the homologous (maternal) locus to assume a heterochromatic structure (black box) and be chromosome was silenced (Duvillie et al. 1998). Physical transcribed (red arrow) while causing the other locus to assume interactions between the two alleles were somehow al- a euchromatic structure (red box) and be transcriptionally si- tered, resulting in the inactivation of the Ins2 promoter lent. The Xist promotor, therefore, resembles the promotor of on the maternal allele. the rolled gene, which is expressed in heterochromatin and si- In view of the demonstrations of homologous pairing lent in euchromatin (Eberl et al. 1993). (A) Imprinting (triangles) in the extraembryonic tissue of rodents causes the paternal X and transvection at imprinted regions in mammals, I chromosome to always be the one that is inactivated. Note that propose that transvection is responsible for X-chromo- the imprint(s) may not be entirely on the maternal X chromosome counting, the random choosing of one X chromosome as diagrammed. (B) In the absence of imprinting, homolo- some, and the initiation of heterochromatin formation gous pairing leads to the random initiation of X-inactivation. during X-inactivation. In this scenario, physical contact between the two Xist loci of female cells causes one Xist One possibility is that the chromosomes only pair at locus to be remodeled into heterochromatin while the their heterochromatic repeats. The ability of heterochro- other Xist locus remains euchromatic. Imprinted X-in- matin to associate homologously is well documented activation, like autosomal imprinting, proceeds by the (Lica et al. 1986; Cook and Karpen 1994; Dernburg et same mechanism (Fig. 1A) with the additional presence al. 1996b). The elements that Xist transgenes lack (Heard of an imprint which causes the same allele to always et al. 1999b) would, therefore, be heterochromatic re- form heterochromatin. In the absence of the imprint, peats. The failure of autosomal homologs, homozygous pairing leads to random heterochromatin formation (Fig. for a single copy ectopic Xist locus, to initiate inactiva- 1B). tion (Heard et al. 1999b) may be caused by insufficient The pairing hypothesis for X-inactivation is supported patches of heterochromatin around the ectopic Xist loci by the recent finding that Xist transgenes will only in- to facilitate pairing. Similarly, a single copy Xist trans- activate autosomal sequences when they are in tandem gene would have difficulty pairing with the endogenous multicopy arrays (Heard et al. 1999b). The ability of tan- locus. dem multicopy, but not single copy, Xist transgenes to An important distinction between the transvection inactivate autosomal genes can be explained by pairing observed in Drosophila and the interactions proposed interactions between transgene copies that are facili- here is the outcome of the pairing event. In Drosophila, tated by the immediate proximity of the repeats to each transvection between wild type alleles results in the up- other. Homology-driven pairing interactions are thought regulation of both (Tartof and Henikoff 1991). The allelic to be responsible for the potent heterochromatin-form- interactions proposed here result in one allele forming ing ability of many tandem repeats (Selker 1999). Tan- heterochromatin. dem inverted repeats, which simply need to fold over once to pair, exhibit the most potent heterochromatin- A repeat hypothesis for the spread of X-inactivation forming ability (Selker 1999). Why do imprinted regions in mammals homologously The transvection hypothesis fails to address how hetero- associate while other, nonimprinted, regions do not? chromatin manages to spread throughout the entire X

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Marahrens chromosome once it has established itself at one of the The silent Xist allele on the active X chromosome is two Xist loci. An important clue for solving this problem early replicating while the transcribed Xist allele on the presents itself when we inspect X-autosome transloca- inactive X chromosome is late replicating (Hansen et al. tions. The heterochromatin generally fails to spread very 1995; Gartler et al. 1999). The implication that the tran- far into autosomal sequence (Eicher 1970; White et al. scribed gene has properties of heterochromatin was re- 1998). However, in a small subset of chimeric chromo- cently supported by the demonstration that the tran- somes, inactivation readily spreads across large stretches scribed Xist allele fractionates with heterochromatin of autosome (Eicher 1970; Lee and Jaenisch 1997; Lyon while the silent Xist allele fractionates with euchroma- 1998). To account for the differing abilities of hetero- tin (Endo et al. 1999). chromatin spread, Riggs proposed that ‘way stations’ or ‘boosters,’ positioned at intervals along the X chromosome, boost the X-inactivation signal (Riggs 1990). Interactions between noncontiguous heterochromatic Lyon reasoned that the boosters could be repetitive regions stimulate heterochromatin to spread sequences (Lyon 1998). These studies compared the se- Analyses of the conditions that influence the spread of quence content of the chromosomal regions that permit heterochromatin in Drosophila have uncovered proper- the spread of heterochromatin to the regions refractory ties of heterochromatin that, if applied to X-inactivation, to the spread of heterochromatin (Lyon 1998). This com- provide a straightforward and logical explanation of how parison uncovered a striking correlation: those chromo- Xist can cause heterochromatin to spread throughout a somal regions that promote the long distance spread of 160-Mb chromosome. The studies revealed that two heterochromatin were also those regions that featured large but noncontiguous regions of heterochromatic re- high concentrations of LINE transposable elements (Ka- petitive sequence on a chromosome physically associ- zazian and Moran 1998; Lyon 1998). The X chromosome ated with each other (Csink and Henikoff 1996; Dern- was the only chromosome with a high concentration of burg et al. 1996a). This association was shown to boost LINE elements throughout its entire length (Boyle et al. the distance that heterochromatin spreads from one re- 1990). The autosomal regions refractory to the spread of petitive region into the flanking nonrepetitive regions heterochromatin were barren of LINE elements (Lyon (Henikoff et al. 1995). For this boosting effect to occur, 1998) as was the pseudoautsomal region which escapes the two blocks of heterochromatin were required to be X-inactivation (Boyle et al. 1990). This remarkable find- within a certain threshold distance of each other (Heni- ing suggests that Xist cooperates with LINE elements to koff et al. 1995). Applying this property of heterochro- spread heterochromatin throughout the X chromosome. matin to the X chromosome, if the heterochromatin that Approximately 100,000 LINE elements are dispersed forms at and around one Xist allele physically associates throughout mammalian chromosomes, constituting with heterochromatin elsewhere on the X chromosome, ∼15% of the mammalian genome (Smit 1996) and posing it would stimulate the heterochromatin to spread. Be- a potential threat to genome stability. An elegant screen- cause LINE elements are dispersed at a high density ing strategy for active elements revealed that numerous throughout the mammalian X chromosome, they pro- LINE elements in the human genome are potentially ac- vide sites from which heterochromatin can spread into tive (Sassaman et al. 1997) and could cause gene disrup- the chromosomal regions between the elements until tion if they transposed. LINE elements can also cause the entire X chromosome is heterochromatic. unwanted LINE–LINE recombination. LINE elements The X-inactivation picture that emerges is as follows. appear not to do more damage because they are kept In response to a developmental signal (Fig. 2A), physical inactive by heterochromatin formation. The highest interactions between the two Xist alleles cause hetero- concentrations of LINE elements are found in visible chromatin to form at one allele, but not at the other heterochromatin regions while more isolated elements, allele (Fig. 2B). The heterochromatin that forms at and although not nested in cytologically visible heterochro- around the Xist allele physically interacts with other matin, have been shown to be heavily methylated (Yoder heterochromatin on the same chromosome (LINE ele- et al. 1997) suggesting localized heterochromatin re- ments shown) stimulating the heterochromatin to gions. Presumably this reflects the ability, which has spread further into the adjoining euchromatic regions been demonstrated in a variety of organisms (Pal-Bhadra (Fig. 2C). Interactions with other heterochromatin re- et al. 1997; Matzke et al. 1994; Matzke and Matzke gions induces spreading until X-inactivation is complete 1995a,b,c; Roche and Rio 1998; Jensen et al. 1999; Selker (Fig. 2D). The incomplete or variable spread of hetero- 1999), of dispersed transposons and dispersed repetitive chromatin throughout a euchromatic region results in transgenes, to induce heterochromatin. the genes in this region completely or partially escaping An important clue as to how Xist might be interacting X-inactivation. with LINE elements comes from recent studies into the chromatin structure of the Xist locus. It had always been assumed that the transcribed Xist allele on the inactive Chromatin effects and nonrandom X-inactivation X chromosome is euchromatic. However, a closer exami- nation of the replication timing of the active and inac- The model for X-inactivation helps explain how different tive Xist alleles revealed that for Xist, the relationship XIST mutations might have opposite effects on X-chro- between transcription and replication timing is reversed. mosome choosing. One 15-kb deletion in the Xist locus

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X-inactivation by chromosomal pairing

cilitate heterochromatinization when X-inactivation oc- curs. If flanking regions influence the chromatin state of Xist prior to X-inactivation, then allelic differences in the effect on the chromatin structure of Xist may result in the preferential inactivation of the X chromosome of one allele over the X chromosome of the other allele. A difference in the size or proximity of nearby heterochromatin might explain the influence of the X-controling element (Xce), which maps just 3Ј to Xist (Heard et al. 1999a), on X-chromosome choosing. Heterozygosity at the Xce locus results in the X chromosome bearing one Xce allele that is preferentially inactivated over the X chromosome carrying the other allele (Cattanach and Williams 1972).

Medical implications The view of X-inactivation presented here highlights interactions that may also play a role in patients with mutations that are hundreds of kilobases away from an intact, but misexpressed, disease gene (for review see, Figure 2. Model for X inactivation. (A) A developmental sig- Kleinjan and van Heyningen 1998). Among the most ex- nal, in conjunction with homologous pairing of the two Xist tensively studied are long-distance mutations involving alleles, is responsible for the chromatin remodeling that fol- the imprinted gene cluster implicated in PWS and AS. In lows. (B) One Xist allele assumes a heterochromatic struc- some patients with PWS the causative mutations disturb ture and the other allele a euchromatic structure. (C) Hetero- gene expression over megabases, while in other patients chromatin encompassing the Xist locus physically associates the causative mutations may be even tens of megabases with other heterochromatic regions, stimulating the inactive away from disease genes (Glenn et al. 1993). In several D chromatin structures to spread into adjacent regions. ( ) Het- unrelated patients with X-linked deafness, misexpres- erochromatin association induces spreading from multiple het- sion of the intact POU34 gene is caused by a minidele- erochromatin sites until inactivation is complete. (Yellow light- ening) Developmental signal; (brown jagged regions) hetero- tion >400 kb upstream of the gene (de Kok et al. 1995a,b) chromatin; (black arrows) the direction of heterochromatin while five additional patients with the same disease have spreading; (red box) transcriptionally silent Xist allele; (black microdeletions 900 kb upstream of the disease-causing box) actively transcribed Xist allele; (gray boxes) individual het- gene (de Kok et al. 1996). Mouse mutations that influ- erochromatic LINE elements or clusters of heterochromatic ence gene expression over long distances have also been LINE elements. identified (Cordes and Barsh 1994; Miller et al. 1994; Bedell et al. 1995). Some of the mutations that act over long distances are known to remove DNA repeats (Klein- causes the wild type X chromosome to be inactivated jan and van Heyningen 1998) suggesting that the pairing preferentially in female embryos (Marahrens et al. 1998). of repetitive sequences may bring distant sites together. I infer that this deletion does not prevent homologous Progress in understanding how pairing and other long pairing because X-inactivation does occur. However, the distance interactions influence gene expression may mutation modifies transvection causing the wild type prove useful for the development of therapies for pa- allele to be epigenetically altered. Since homologous tients suffering from long distance mutations and for the pairing should cause the intact Xist gene to loop out, the development of gene therapy in general. potential of the intact Xist allele to interact with heterochromatin on the wild type X chromosome might be Note added in proof increased. Another targeted deletion removes Ն3.1 kb from the 3Ј portion of the Xist gene (Hong et al. 1999) and Lossi et al. (Am. J. Hum. Genet. 1999. 65: 558–562) have re- an additional 62 kb 3Ј of the gene (Clerc and Avner 1998). cently reported an X-inactivation defect in patients with a mu- This second deletion causes the X chromosome bearing tation in the ATR-X gene. The ATR-X gene product interacts with the homolog of a Drosophila protein known to be impor- the deletion to always be inactivated, even when it is the tant for transvection. only X chromosome in the cell (Clerc and Avner 1998). The second deletion clearly does not abolish the ability of the mutant Xist locus to induce X-inactivation. Per- Acknowledgments haps this 65-kb deletion causes a nearby region of het- I thank Laura Gammill, Eric Vilain, and Krzys Stanczak for erochromatin to be closely juxtaposed with Xist, making comments on the manuscript and Arnie Berk for helpful discus- the Xist locus more heterochromatic. A more hetero- sions, and the numerous people who helped me to write this chromatic state of Xist prior to X-inactivation may fa- manuscript.

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Marahrens

References Davis, and B.J. Meyer. 1999. Dosage compensation proteins targeted to X chromosomes by a determinant of hermaphro- Baker, B.S., M. Gorman, and I. Marin. 1994. Dosage compensa- dite fate. Science 284: 1800–1804. Drosophila Annu. Rev. Genet. tion in . 28: 491–521. de Kok, Y.J.M., G.F.M. Merkx, S.M. van der Maarel, I. Huber, S. Bartolomei, M.S. and S.M. Tilghman. 1997. Genomic imprint- Malcolm, H.-H. Ropers, and F.P.M. Cremers. 1995a. A du- Annu. Rev. Genet. ing in mammals. 31: 493–525. plication/pancreatic inversion associated with familial X- Bartolomei, M.S., S. Zemel, and S.M. Tilghman. 1991. Parental linked deafness (DFN3) suggests the presence of a regulatory H19 Nature imprinting of the mouse gene. 351: 153–155. element more than 400 kb upstream of the POU3F4 gene. Bartolomei, M.S., A.L. Webber, M.E. Brunkow, and S.M. Tilgh- Hum. Mol. Genet. 4: 2145–2150. man. 1993. Epigenetic mechanisms underlying the imprint- de Kok, Y.J.M., S.M. van der Maarel, M. Bitner-Glindzicz, I. H19 Genes Dev. ing of the mouse gene. & 7: 1663–1673. Huber, A.P. Monaco, S. Malcolm, M.E. Pembrey, H.-H. Rop- Beard, C., E. Li, and R. Jeanisch. 1995. Loss of methylation ac- ers, and F.P.M. Cremers. 1995b. Association between Xist Genes tivates in somatic but not in embryonic cells. & X-linked mixed deafness and mutations in the POU domain Dev. 9: 2325–2334. gene POU3F4. Science 267: 685–688. Bedell, M.A., C.I. Brannan, E.P. Evans, N.G. Copeland, N.A. de Kok, Y.J.M., E.R. Vossenaar, C.W. Cremers, N. Dahl, J. Jenkins, and P.J. Donovan. 1995. DNA rearrangements lo- Laporte, L.J. Hu, D. Lacombe, N. Fischel-Ghodsian, R.A. steel S1 Steel- cated over 100 kb 5Ј of the ( )-coding region in Friedman, L.S. Parnes et al. 1996. Identification of a hot spot panda Steel-contrasted S1 and mice deregulate expression for microdeletions in patients with X-linked deafness type 3 and cause female sterility by disrupting ovarian follicle de- (DFN3) 900 kb proximal to the DFN3 gene POU3F4. Hum. Genes Dev. velopment. & 9: 455–470. Mol. Genet. 5: 1229–1235. Borsani, G., R. Tonlorenzi, M.C. Simmler, L. Dandolo, D. Ar- DeLorey, T.M., A. Handforth, S.G. Anagnostaras, G.E. Homan- naud, V. Capra, M. Grompe, A. Pizzuti, D. Muzny, C. ics, B.A. Minassian, A. Asatourian, M.S. Fanselow, A. Del- Lawrence, H.F. Willard, P. Avner, and A. Ballabio. 1991. gado-Escueta, G.D. Ellison, and R.W. Olsen. 1998. Mice Characterization of a murine gene expressed from the inac- lacking the beta3 subunit of the GABAA receptor have the Nature tive X chromosome. 351: 325–328. epilepsy phenotype and many of the behavioral characteris- Boyle, A.L., S.G. Ballard, and D.C. Ward. 1990. Differential dis- tics of Angelman syndrome. J. Neurosci. 18: 8505–8514. tribution of long and short interspersed element sequences Dernburg, A.F., K.W. Broman, J.C. Fung, W.F. Marshall, J. Phil- in the mouse genome: Chromosome karyotyping by fluores- ips, D.A. Agard, and J.W. Sedat. 1996a. Perturbation of Proc. Natl. Acad. Sci. cence in situ hybridization. 87: 7757– nuclear architecture by long-distance chromosome interac- 7761. tions. Cell 85: 745–759. Brandeis, M., T. Kafri, M. Ariel, J.R. Chaillet, J. McCarrey, A. Dernburg, A.F., J.W. Sedat, and R.S. Hawley. 1996b. Direct evi- Razin, and H. Cedar. 1993. The ontogeny of allele-specific dence of a role for heterochromatin in meiotic chromosome methylation associated with imprinted genes in the mouse. segregation. Cell 86: 135–146. EMBO J. 12: 3669–3677. Driscoll, D. 1994. Genomic imprinting in humans. Mol. Genet. Brown, C.J., A. Ballabio, J.L. Rupert, R.G. Lafreniere, M. Med. 4: 37–77. Grompe, R. Tonlorenzi, and H.F. Willard. 1991. A gene from Duvillie, B., D. Bucchini, T. Tang, J. Jami, and A. Paldi. 1998. the region of the human X inactivation centre is expressed Imprinting at the mouse Ins2 locus: Evidence for cis-and Nature exclusively from the inactive X chromosome. trans-allelic interactions. Genomics 47: 52–57. 349: 38–44 . Eberl, D.F., B.J. Duyf, and A.J. Hilliker. 1993. The role of het- Brown, C.J., B.D. Hendrich, J.L. Rupert, R.G. Lafreniere, Y. Xing, erochromatin in the expression of a heterochromatic gene, XIST J. Lawrence, and H.F. Willard. 1992. The human gene: the rolled locus of Drosophila melanogaster. Genetics Analysis of a 17 kb inactive X-specific RNA that contains 134: 277–292. conserved repeats and is highly localized within the nucleus. Eicher, E.M. 1970. X-autosome translocations in the mouse: Cell 71: 527–542. Total inactivation versus partial inactivation of the X chro- Cattanach, B.M. and C.E. Williams. 1972. Evidence of nonran- mosome. Adv. Genet. 15: 175–259. Genet. Res. dom X chromosome activity in the mouse. Endo, S. and N. Takagi. 1981. A preliminary cytogenetic study 19: 229–240. of X chromosome inactivation in diploid parthenogenetic Clemson, C.M., J.A. McNeil, H.F. Willard, and J.B. Lawrence. embryos from LT/Sv mice. Jpn. J. Genet. 56: 349–356. XIST 1996. RNA paints the inactive X chromosome at in- Endo, S., N. Takagi, and M. Sasaki. 1982. The late-replicating X terphase: Evidence for a novel RNA involved in nuclear/ chromosome in digynous mouse triploid embryos. Dev. J. Cell Biol. chromosome structure. 132: 259–275. Genet. 3: 165–176. Xist Clerc, P. and P. Avner. 1998. Role of the region 3Ј to exon Endo, Y., T. Watanabe, Y. Mishima, A. Yoshimura, N. Takagi, 6 in the counting process of X-chromosome inactivation. and R. Kominami. 1999. Compact chromatin packaging of Nat. Genet. 19: 249–253. inactive X chromosome involves the actively transcribed Cook, K.R. and G.H. Karpen. 1994. A rosy future for heterochro- Xist gene. Mamm Genome 10: 606–610. Proc. Natl. Acad. Sci. matin. 91: 5219–5221. Fangman, W.L. and B.J. Brewer. 1992. A question of time: Rep- Cooper, D.W. 1971. Directed genetic change model for X chro- lication origins of eukaryotic chromosomes. Cell 71: 363– Nature mosome inactivation in eutherian mammals. 366. 230: 292–294. Ferguson-Smith, A.C., B.M. Cattanach, S.C. Barton, C.V. Cordes, S.P. and G.S. Barsh. 1994. The mouse segmentation Beechey, and M.A. Surani. 1991. Embryological and molecu- kr gene encodes a novel basic domain-leucine zipper tran- lar investigations of parental imprinting on mouse chromo- Cell scription factor. 79: 1025–1034. some 7. Nature 351: 667–670. Csink, A.K. and S. Henikoff. 1996. Genetic modification of het- Garrick, D., S. Fiering, D.I. Martin, and E. Whitelaw. 1998. Re- Dro- erochromatic association and nuclear organization in peat-induced gene silencing in mammals. Nat. Genet. sophila Nature . 381: 529–531. 18: 56–59. Dawes, H.E., D.S. Berlin, D.M. Lapidus, C. Nusbaum, T.L. Gartler, S.M., L. Goldstein, S.E. Tyler-Freer, and R.S. Hansen.

2630 GENES & DEVELOPMENT Downloaded from genesdev.cshlp.org on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press

X-inactivation by chromosomal pairing

1999. The timing of Xist replication: dominance of the do- inactivation center. Cell 86: 83–94. main. Hum. Mol. Genetics 8: 1085–1089. Leighton, P.A., R.S. Ingram, J. Eggenschwiler, A. Efstratiadis, Glenn, C.C., R.D. Nicholls, W.P. Robinson, S. Saitoh, N. Nii- and S.M. Tilghman. 1995. Disruption of imprinting caused by kawa, A. Schinzel, B. Horsthemke, and D.J. Driscoll. 1993. deletion of the H19 gene region in mice. Nature 375: 34–39. Modification of 15q11-q13 DNA methylation imprints in Lewis, E.B. 1954. The theory and application of a new method of unique Angelman and Prader-Willi patients. Hum. Mol. detecting chromosomal rearrangements in Drosophila me- Genet. 2: 1377–1382. lanogaster. Am. Nat. 88: 225–239. Grunstein, M. 1997. Histone acetylation in chromatin structure Li, E., C. Beard, and R. Jaenisch. 1993. Role for DNA methyl- and transcription. Nature 389: 349–352. ation in genomic imprinting. Nature 366: 362–365. Hansen, R.S., T.K. Canfield, and S.M. Gartler. 1995. Reverse Lica, L.M., S. Narayanswami, and B.A. Hamkalo. 1986. Mouse replication timing for the XIST gene in human fibroblasts. satellite DNA, centromere structure, and sister chromatid Hum. Mol. Genet. 4: 813–820. pairing. J. Cell Biol. 103: 1145–1151. Heard, E., R. Lovell-Badge, and P. Avner. 1999a. Anti-Xisten- Lyon, M.F. 1998. X-chromosome inactivation: A repeat hypoth- tialism. Nat. Genet. 21: 343–344. esis. Cytogenet. Cell Genet. 80: 133–137. Heard, E., F. Mongelard, D. Arnaud, and P. Avner. 1999b. Xist Marahrens, Y., J. Loring, and R. Jaenisch. 1998. Role of the Xist yeast artificial chromosome transgenes function as X-inac- gene in X chromosome choosing. Cell 92: 657–664. tivation centers only in multicopy arrays and not as single Marahrens, Y., B. Panning, J. Dausman, W. Strauss, and R. Jae- copies. Mol. Cell. Biol. 19: 3156–3166. nisch. 1997. Xist-deficient mice are defective in dosage com- Heitz, E. 1928. Das heterochromatin der moose. IJahrbWiss pensation but not spermatogenesis. Genes & Dev. 11: 156– Botanik 69: 762–818. 166. Henikoff, S. 1997. Nuclear organization and gene expression: Matzke, A.J. and M.A. Matzke. 1995a. Trans-inactivation of ho- homologous pairing and long-range interactions. Curr. Op. mologous sequences in Nicotiana tabacum. Curr. Top. Mi- Cell Biol. 9: 388–395. crobiol. Immunol. 197: 1–14. Henikoff, S., J.M. Jackson, and P.B. Talbert. 1995. Distance and Matzke, M., A.J.M. Matzke, and O.M. Schied. 1994. Inactiva- pairing effects on the brownDominant heterochromatic ele- tion of repeated genes–DNA–DNA interaction? In Homolo- ment in Drosophila. Genetics 140: 1007–1017. gous recombination and gene silencing in plants (ed. J. Pasz- Herzing, L.B.K., J.T. Romer, J.M. Horn, and A. Ashworth. 1997. kowski) pp. 271–307. Kluyver Academic Publishers, Nether- Xist has properties of the X-chromosome inactivation cen- lands. tre. Nature 386: 272–275. Matzke, M.A. and A.J.M. Matzke. 1995b. Homology-dependent Hollander, G.A., S. Zuklys, C. Morel, E. Mizoguchi, K. Mobis- gene silencing in transgenic plants: What does it really tell son, S. Simpson, C. Terhorst, W. Wishart, D.E. Golan, A.K. us? Trends Genet. 11: 1–3. Bhan, and S.J. Burakoff. 1998. Monoallelic expression of the Matzke, M.A. and A.J.M. Matzke. 1995c. How and why do interleukin-2 locus. Science 279: 2118–2121. plants inactivate homologous (trans) genes? Plant Physiol. Hong, Y.K., S.D. Ontiveros, C. Chen, and W.M. Strauss. 1999. A 107: 679–685. new structure for the murine Xist gene and its relationship Metz, C.W. 1916. Chromosome studies on the Diptera II. The to chromosome choice/counting during X-chromosome in- paired association of chromosomes in the diptera, and its activation. Proc. Natl. Acad. Sci. 96: 6829–6834. significance. J. Exp. Zool. 21: 213–280. Jensen, S., M.-P. Gassama, and T. Heidemann. 1999. Taming of Meyer, P. and H. Saedler. 1996. Homology-dependent gene si- transposable elements by homology-dependent gene silenc- lencing in plants. Annu. Rev. Plant Physiol. Plant Mol. Biol. ing. Nat. Genet. 21: 209–212. 47: 23–48. Jeppesen, P. and B.M. Turner. 1993. The inactive X chromosome Migeon, B.R. 1990. Insights into X chromosome inactivation in female mammals is distinguished by a lack of histone H4 from studies of species variation, DNA methylation and rep- acetylation, a cytogenetic marker for gene expression. Cell lication, and vice versa. Genet. Res. 56: 91–98. 74: 281–289. Miller, M.W., D.M. Duhl, B.M. Winkes, F. Arredondo-Vega, P.J. Jones, B.K., J.M. Levorse, and S.M. Tilghman. 1998. Igf2 im- Saxon, G.L. Wolff, C.J. Epstein, M.S. Hershfield, and G.S. printing does not require its own DNA methylation or H19 Barsh. 1994. The mouse lethal nonagouti (ax) mutation de- RNA. Genes & Dev. 12: 2200–2207. letes the S-adenosylhomocysteine hydrolase (Ahcy) gene. Karpen, G.H. 1994. Position-effect variegation and the new bi- EMBO J. 13: 1806–1816. ology of heterochromatin. Curr. Biol. 4: 281–291. Mohandas, T., R.S. Sparkes, and L.J. Shapiro. 1981. Reactivation Kazazian, H.H.J. and J.V. Moran. 1998. The impact of L1 retro- of an inactive human X chromosome: Evidence for X inac- transposons on the human genome. Nat. Genet. 19: 19–24. tivation by DNA methylation. Science 211: 393–396. Kitsberg, D., S. Selig, M. Brandeis, I. Simon, I. Keshet, D.J. Mutirangura, A., A. Jayakumar, J.S. Sutcliffe, M. Nakao, M.J. Driscoll, R.D. Nicholls, and H. Cedar. 1993. Allele-specific McKinney, K. Buiting, B. Horsthemke, A.L. Beaudet, A.C. replication timing of imprinted gene regions. Nature Chinault, and D.H. Ledbetter. 1993. A complete YAC contig 364: 459–463. of the Prader-Willi/Angelman chromosome region (15q11- Klein, C.B. and M. Costa. 1997. DNA methylation, heterochro- q13) and refined localization of the SNRPN gene. Genomics matin and epigenetic carcinogens. Mut. Res. 386: 163–180. 18: 546–552. Kleinjan, D.-J. and V. van Heyningen. 1998. Position effect in Mutter, G.L., C.L. Stewart, M.L. Chaponot, and R.J. Pomponio. human genetic disease. Hum. Mol. Genet. 7: 1611–1618. 1993. Oppositely imprinting genes H19 and insulin-like LaSalle, J.M. and M. Lalande. 1996. Homologous association growth factor 2 are coexpressed in human androgenetic tro- of oppositely imprinted chromosomal domains. Science phoblast. Am. J. Hum. Genet. 53: 1096–1102. 272: 725–728. Nicoll, M., C.C. Akerib, and B.J. Meyer. 1997. X chromosome Lee, J.T. and R. Jaenisch. 1997. Long-range cis effects of ectopic counting mechanisms that determine nematode sex. Nature X-inactivation centres on a mouse autosome. 386: 275–279. 388: 200–204. Lee, J.T., W.M. Strauss, J.A. Dausman, and R. Jaenisch. 1996. A Norris, D.P., D. Patel, G.F. Kay, G.D. Penny, N. Brockdorff, S.A. 450 kb transgene displays properties of the mammalian X- Sheardon, and S. Rastan. 1994. Evidence that random and

GENES & DEVELOPMENT 2631 Downloaded from genesdev.cshlp.org on September 30, 2021 - Published by Cold Spring Harbor Laboratory Press

Marahrens

imprinted Xist expression is controlled by preemptive meth- a differentially methylated CpG island at the SNRPN gene ylation. Cell 77: 41–51. define a putative imprinting control region. Nat. Genet. Ohlsson, R., F. Flam, R. Fisher, S. Miller, H. Cui, S. Pfeifer, and 8: 52–58. G.I. Adam. 1999. Random monoallelic expression of the im- Svensson, K., R. Mattsson, T.C. James, P. Wentzel, M. Pilartz, J. printed Igf2 and H19 genes in the absence of discriminative MacLaughlin, S.J. Miller, T. Olsson, U.J. Eriksson, and R. parental marks. Dev. Genes Evol. 209: 113–119. Ohlsson. 1998. The paternal allele of the H19 gene is pro- Ohno, S. and T.S. Hauschka. 1960. Allocycly of the X chromo- gressively silenced during early mouse development: The some in tumors and normal tissues. Cancer Res. 20: 541– acetylation status of histones may be involved in the gen- 545. eration of variegated expression patterns. Development O’Neill, L.P., A.M. Keohane, J.S. Lavender, V. McCabe, E. 125: 61–69. Heard, P. Avner, N. Brockdorff, and B.M. Turner. 1999. A Tada, T. and N. Takagi. 1992. Early development and X chro- developmental switch in H4 acetylation upstream of Xist mosome inactivation in mouse parthenogenetic embryos. plays a role in X chromosome inactivation. EMBO J. Mol. Reprod. Dev. 31: 20–27. 18: 2897–2907. Takagi, N. and M. Sasaki. 1975. Preferential inactivation of the Pal-Bhadra, M., U. Bhadra, and J.A. Birchler. 1997. Cosuppres- paternally derived X chromosome in the extraembryonic sion in Drosophila: Gene silencing of alcohol dehydrogenase membranes of the mouse. Nature 256: 640–642. by white-ADH transgenes is Polycomb dependent. Cell Tartof, K.D. and S. Henikoff. 1991. Trans-sensing effects from 90: 479–490. Drosophila to humans. Cell 65: 201–203. Panning, B. and R. Jaenisch. 1996. DNA hypomethylation can Taylor, J.H. 1960. Asynchronous duplication of chromosomes activate Xist expression and silence X-linked genes. Genes & in cultured cells of Chinese hamster. J. Biophys. Biochem. Dev. 10: 1991–2002. Cytol. 7: 455–464. Penny, G.D., G.F. Kay, S.A. Sheardown, S. Rastan, and N. Brock- Turner, B.M. 1998. Histone acetylation as an epigenetic deter- dorff. 1996. Requirement for Xist in X chromosome inacti- minant of long-term transcriptional competence. Cell. Mol. vation. Nature 379: 131–137. Life Sci. 54: 21–31. Pfeifer, K. and S.M. Tilghman. 1994. Allele-specific gene expres- Walsh, C., A. Glaser, R. Fundele, A. Ferguson-Smith, S. Barton, sion in mammals: The curious case of the imprinted RNAs. M.A. Surani, and R. Ohlsson. 1994. The nonviability of uni- Genes & Dev. 8: 1867–1874. parental mouse conceptuses correlates with the loss of the Rachmilewitz, J., R. Goshen, I. Ariel, T. Schneider, N. de Groot, products of imprinting genes. Mech. Dev. 46: 55–62. and A. Hochberg. 1992. Parental imprinting of the human White, W.M., H.F. Willard, D.L. Van Dyke, and D.J. Wolff. 1998. H19 gene. FEBS Lett. 309: 25–28. The spreading of X inactivation into autosomal material of Rastan, S., M.H. Kaufman, A.H. Handyside, and M.F. Lyon. an X autosome translocation: Evidence for a difference be- 1980. X chromosome inactivation in extraembryonic mem- tween autosomal and X-chromosomal DNA. Am. J. Hum. branes of diploid parthenogenetic mouse embryos demon- Genet. 63: 20–28. strated by differential staining. Nature 288: 172–173. Yoder, J.A., C.P. Walsh, and T.H. Bestor. 1997. Cytosine meth- Riggs, A.D. 1990. Marsupials and mechanisms of X chromo- ylation and the ecology of intragenomic parasites. Trends some inactivation. Aust. J. Zool. 37: 419–441. Genet. 13: 335–340. Ripoche, M.A., C. Kress, F. Poirier, and L. Dandolo. 1997. De- Zhang, Y. and B. Tycko. 1992. Monoallelic expression of the letion of the H19 transcription unit reveals the existence of human H19 gene. Nat. Genet. 1: 40–44. a putative imprinting control element. Genes & Dev. 11: 1596–1604. Roche, S.E. and D.C. Rio. 1998. Trans-silencing by P elements inserted in subtelomeric heterochromatin involves the drosophila polycomb group gene, enhancer of zeste. Genetics 149: 1839–1855. Sasaki, H., A.C. Ferguson-Smith, A.S.W. Shum, S.C. Barton, and M.A. Surani. 1995. Temporal and spatial regulation of H19 imprinting in normal and uniparental mouse embryos. Development 121: 4195–4202. Sassaman, D.M., B.A. Dombroski, J.V. Moran, M.L. Kimber- land, T.P. Naas, R.J. DeBerardinas, A. Gabriel, G.D. Swer- gold, and H.H.J. Kazazian. 1997. Many human L1 elements are capable of retrotransposition. Nature Genet. 16: 37–43. Selker, E. 1999. Gene silencing: Repeats that count. Cell 97: 157–160. Selker, E.U. 1990. DNA methylation and chromatin structure: A view from below. Trends Biochem. Sci. 15: 103–107. Shao, C. and N. Takagi. 1990. An extra maternally derived X chromosome is deleterious to early mouse development. De- velopment 110: 969–975. Sharman, G.B. 1971. Late DNA replication in the paternally derived X chromosome of female kangaroos. Nature 230: 231–232. Smit, A.F. 1996. The origin of interspersed repeats in the human genome. Curr. Opin. Genet. Dev. 6: 743–748. Sutcliffe, J.S., M. Nakao, S. Christian, K.H. Orstavik, N. Tom- merup, D.H. Ledbetter, and A.L. Beaudet. 1994. Deletions of

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